Intra-tidal bed evolution on a macrotidal dissipative beach during a storm: contrasting roles of extremely shallow and relatively deep water stages

Abstract. Intra-tidal morphodynamics on macrotidal dissipative beaches remain poorly understood due to challenges in capturing continuous bed-level evolution under shallow-water conditions. Here, we used a high-resolution vertical probe (Argus Surface Meter, ASM) to obtain minute-scale, continuous records of bed elevation and near-bed suspended sediment concentration (SSC) in the intertidal zone across Extremely Shallow Water Stages (ESWS, water depths < 0.3 m) and Relatively Deep Water Stages (RDWS, water depths > 0.3 m) during storm attenuation. Our results reveal three key findings: (1) The relative dominance of ESWS and RDWS in bed-level evolution reverses as storm decays. RDWS drives the most significant changes under storm conditions, while the contribution of ESWS increases substantially as storm decays, becoming predominant in fair weather when bed shear stress falls below the critical threshold in RDWS. (2) A consistent flood-phase erosion–ebb-phase accretion pattern characterizes ESWS, whereas RDWS exhibits state-dependent behavior: under storms, this pattern is maintained by tide-modulated sediment supply—with active scouring during flood due to high transport capacity and deposition during ebb due to overwhelming sediment delivery from the migrating breaker zone. In fair weather, it reverses to flood-accretion/ebb-erosion due to settling of sediment during flood and subsequent resuspension during ebb. (3) the interplay between ESWS and RDWS highlights the critical role of complete ESWS process in sustaining dissipative beach stability. Based on these findings, we propose a conceptual model integrating ESWS and RDWS, providing a basis for predicting intertidal morphodynamic evolution in tide-dominated systems under changing storms.